Biometric authentication (or biometrics authentication) identifies a user using biometric features of the user, such as fingerprint, face, vein, or the like of the user. Vein authentication captures subcutaneous vein patterns to acquire the biometric features. Because the vein authentication identifies the user using internal information, the vein authentication provides an authentication accuracy that is higher compared to those provided by authentications that identify the user using information such as the fingerprint, face, or the like.
In order to perform the vein authentication, light is irradiated on a biometric part to capture the vein pattern. For example, the light irradiated on the biometric light may be near-infrared light that penetrates skin and reaches inside the biometric part. The biometric part, such as a finger, hand, or the like, has a multi-layered structure including skin and internal structure. Hence, an appearance of the biometric part may be represented by a dichroic reflection model. In other words, in a case in which light is irradiated on the biometric part, returning light from the biometric part is a mixture of light reflected at a surface of the biometric part (that is, skin surface) and light scattered inside the biometric part. Because veins are located under the skin, an image caused by surface reflection is eliminated, and only an image caused by internal scattering is obtained, in order to observe the veins with a high accuracy.
The image caused by surface reflection and the image caused by internal scattering may be separated using polarization properties. For example, Japanese Laid-Open Patent Publication No. 2002-200050 proposes a technique that uses a polarization filter to eliminate effects of surface reflection. The light reflected at the surface of the biometric part maintains the polarization state thereof. On the other hand, the light scattered inside the biometric part randomly change the polarization state thereof. For this reason, when the polarization filter is arranged at a subsequent stage of an illumination end and at a preceding stage of an observation end so that the polarization directions at the illumination end and the observation end become parallel, the polarization filters cut an internally scattered light component, and a surface reflection light component can be observed by a detector. On the other hand, when the polarization filter is arranged at the subsequent stage of the illumination end and at the preceding stage of the observation end so that the polarization directions at the illumination end and the observation end become perpendicular, the polarization filters cut the surface reflection light component, and the internally scattered light component can be observed by the detector.
However, the polarization filter transmits only the light having a particular polarization direction, and cuts light having polarization directions other than the particular polarization direction by reflecting or absorbing the light having polarization directions other than the particular polarization direction. Consequently, an amount of light that can be received by the detector via the polarization filter decreases, and image noise increases, thereby making it difficult to obtain a clear biometric image. In addition, a polarization filter that can be used in a near-infrared range is expensive. As a result, when providing the polarization filter that can be used in the near-infrared range, a cost of the biometric image processing apparatus increases.
Therefore, it is difficult to obtain a clear biometric image by a conventional biometric image processing apparatus.
Examples of related art include Japanese Laid-Open Patent Publications No. 2002-200050, No. 2007-323389, and No. 2009-028427, Takaaki Maeda et al., “Monte Carlo Simulation of Spectral Reflectance Using a Multilayered Skin Tissue Model”, Optical Review Vol. 17, No. 3, (2010), pp. 223-229, and Yoshinaga Aizu, “Skin Tissue Multilayered Structure Modeling and Light Propagation Simulation”, Journal of the Japan Society of Mechanical Engineers (JSME), 2011.7, Vol. 114, No. 1112, p. 39.